U.S. patent application Ser. No. 07/430,037 to Rodenberg et al, filed Nov. 1, 1989, shows an image-forming apparatus in which a series of electrostatic images are formed on an image member. The electrostatic images are toned by application of toner to form toner images, and the toner images are transferred to a receiving sheet. Each receiving sheet is fed to a fuser where its toner image is fixed. In a duplex mode, from the fuser the sheets are fed along a duplex path back to a position upstream of the transfer station from which they are then fed through the system again to receive a toner image on the opposite side. The sheet is again passed through the fuser to fuse the second image and exits the apparatus with fixed images on both sides.
This apparatus also has the ability to make multicolor images on each side of a receiving sheet. To accomplish this, a plurality of toning stations are provided to tone a series of electrostatic images with different color toners. The receiving sheet is wrapped around a transfer drum at the transfer station and rotated through transfer relation with successive images on the image member which are transferred to the receiving sheet in registration to form a multicolor image.
Once the transfer is completed, the receiving sheet is allowed to follow the image member rather than stay on the transfer drum, and it continues on to the fuser where the multicolor image is fused. As with single color images, the sheet may be returned to receive a single color or multicolor image on its oppoisite side and is again passed through the fuser. To make it easier to separate the transfer sheet from the transfer drum, the surface of the transfer drum is substantially roughened to a point where it has a texture of between 0.002 to 0.005 inches separation between peaks and valleys.
In testing of the above apparatus, it provided high quality images, both single color and multicolor. However, on occasion, some images had an unacceptable amount of background, which background was patterned. This image defect occurred now and then in both simplex and duplex copies, both multicolor and single color, but did not always occur.
Obviously, it became desirable to eliminate this imperfection in the image background.
U.S. Pat. No. 4,429,990, issued Feb. 7, 1984 to E. J. Tamary, shows a pressure roller fuser of the type presently commercially used to fix toner images to support sheets. An important aspect of that disclosure is an applicator for applying release liquid to a fusing roller which contacts the toner image. The applicator, commonly called a rotating wick, includes a hollow, porous roller which is supplied with fusing oil internally. The applicator has an inner supply tube with holes in it and is covered by a porous material having a surface of wool or a heat resistant synthetic wicking material. The applicator is rotatable by the fusing roller. The applicator is movable into and out of engagement with the roller according to a program which prevents excess buildup of oil on the roller, which otherwise would stain the receiving sheet.
U.S. Pat. No. 4,549,803 to Ohno et al, issued Oct. 29, 1985 and U.S. Pat. No. 4,593,922 to Yoshinaga et al, issued Jun. 10, 1986, both show fixing devices in which fixing conditions are changed between paper stock and transparency stock to reduce the amount of oil applied when transparencies are being fixed.
U.S. Pat. No. 4,942,433 to Stuart, issued Jul. 17, 1990, shows a wicking device similar to that shown in the Tamary patent. When transparency stock is being fixed, a brake prevents rotation of the wick by the fuser to reduce the amount of oil applied.
U.S. Pat. No. 4,920,382 to Mills et al, issued Apr. 24, 1990, also shows a wicking device similar to that disclosed in Tamary in which a relatively complicated algorithm is used for transparencies, including feeding a sheet of paper through the fuser between transparencies to reduce and smooth the oil on the fusing roller.